When a cooling liquid is injected into a duct through which a hot cracked gas is passing, in a manner such that the liquid contacts the inner surface of the duct wall randomly or non-symmetrically, such introduction of the coolant does not achieve a uniform temperature with respect to a cross-section of the duct at that location. The temperature should be substantially uniform around a cut, in a single plane, of the duct wall; otherwise thermal stresses result which, if they are high enough, cause permanent deformation of the metal wall. Additionally, there are wet and dry areas which fluctuate so that, in dry areas which have previously been wetted and where some drops of liquid remain, polymerization can take place with formation of coke deposits. Such coke deposits increase the pressure drop across the quench pipe, resulting in a reduced selectivity to the desired olefins.
In U.S. Pat. No. 4,121,908, cooling oil is introduced into an annular space between vertical coaxial pipes at a location where the cracked gas has not yet emerged, since the cracked gas outlet is downstream of the quench oil inlets. Thus, the outer duct, at the location of an annular gap, receives the cooling oil which cools the wall of the inner pipe. Consequently, hot cracked products of high molecular weight, e.g., from gas oil feeds, can deposit on the inner surface of the inner pipe. The cracked effluent at that location has not yet come in contact with the quench oil which could act as a flux for the tarry products. Also, the oil is introduced between two pipes and thus is subject to frictional forces from both sides. This creates a drag and slows down the swirling or spiral motion of the oil. Additionally, the quench point described is meant to operate with gas/quench oil flow downward whereas in the subject quench point flow can be in any direction.
U.S. Pat. No. 3,593,968 discloses an apparatus in which nozzles spray quench liquid into a downwardly flowing stream of cracked gas and separate means are used to direct a film of quench liquid downwardly on the wall of the chamber through which the gas is passing. This system is therefore limited in that only a vertical downflow arrangement can be employed.